The present invention relates to a twin-wire forming section of a paper machine.
The twin-wire forming section comprises a lower wire loop, like a modified forming section of a Fourdrinier part to be modernized, defining an initial single-wire portion of the dewatering zone. This is followed by an upper wire loop which, together with the coinciding upper run of the wire of the lower wire loop, defines a twin-wire dewatering zone. Dewatering occurs in the twin-wire dewatering zone through the wires of both the lower wire loop and the upper wire loop.
In known twin-wire formers commonly used in gap formers, the stock is fed onto the wire section as a lean pulp slurry. Then, either immediately, or after a short single-wire run, a violent dewatering from the stock web begins in both directions, or in the same direction as in the single-wire portion. As a result, fillers such as, for example, kaolin added to the stock and also fine fibers, escape in a considerable amount with the water. This, of course, noticeably impairs the quality of the paper web and particularly impairs those properties obtaining which are sought in the paper web by the use of fillers. Furthermore, a simultaneous and violent two-sided dewatering easily causes a weakening of the middle layer of the web, which leads to a low internal bond strength.
Indicative of the state of the art relating to the present invention is U.K. patent application No. 2,003,952 of Beloit Walmsley Limited, UK and U.S. Pat. No. 3,846,233 of the present inventor and U.S. Pat. No. 4,472,244 of the present inventor.
U.S. Pat. No. 3,946,233 discloses a wire section of a paper machine comprising an initial single-wire portion, followed by a twin-wire portion. The twin-wire portion is curved at least as its initial part, guided by a dewatering roller or a dewatering box. In this known wire section, the initial single-wire portion is so long that while the dewatering cautiously occurs in the initial part, the stock web has time prior to entering the twin-wire portion to attain such a felting degree that the fibers are no longer able to move appreciably with reference to each other. Furthermore, the twin-wire section guided by the dewatering roller or by the dewatering box is curved downwards so that the dewatering in this curved portion principally occurs, due to the effect of centrifugal force, through the upper wire and in a direction opposite that in the initial single-wire portion. The object is to reduce the escape of the additives, such as fillers and fine substances of the stock, from the stock web and to improve the internal bond strength in the paper to be produced.
U.S. Pat. No. 4,472,244 discloses a wire section of a paper machine in which a twin-wire dewatering zone on the side of the upper wire unit consists of a curved forming shoe having an unperforated cover. The forming shoe is preceded by a curved dewatering zone which has a forming roller or equivalent, as a shoelike means. The dewatering occurs primarily through the upper wire in the dewatering zone. The water, which is removed from the web through the upper wire, is conducted to the side or sides of the paper machine, substantially without using suction energy, by devices of the upper wire unit, or units, which are positioned above the forming shoe.
Suction rollers or boxes are used in several twin-wire formers of the prior art. The rotating suction rollers are expensive components. Furthermore, both the suction rollers and suction boxes consume a considerable magnitude of suction energy. Also, the apparatus is rather complicated in structure, and it is desirable to improve operating safety.